Energy harvesting recently attracting attention is a power generation technology by collecting micro energy in our surroundings, which is discarded without usage, from heat, vibration, electromagnetic waves, etc. For example, vibration power generation that generates power by applying distortion to a piezoelectric device and radio wave power generation that converts radio waves in the environments into electric power by an antenna are listed. To supply electric energy generated by energy harvesting to a load circuit, an AC-DC converter is used to generate a DC output voltage of desired potential from an AC power source generated by the energy harvesting.
For example, the AC-DC converter includes a power circuit such as a DC-DC converter and a rectifier circuit. First, the rectifier circuit rectifies an AC voltage of the AC power source to a DC voltage. Then, the power circuit such as the DC-DC converter generates an output voltage of desired potential from the above DC potential.
If the output voltage reaches the desired potential, the AC voltage continues to be supplied to the AC-DC converter, as long as the AC power source continues to generate power. As a result, the DC voltage input from the rectifier circuit to the power circuit continues to increase, and produces an overvoltage state. To prevent this, an overvoltage protection circuit is provided in the power circuit, so that the DC voltage input to the power circuit is clamped to a predetermined voltage.
Followings are related prior arts.
The official gazette of the Japanese Laid-open Patent Publication No. 2000-308352.
The official gazette of the Japanese Laid-open Patent Publication No. Hei-10 (1998)-322934.
The official gazette of the Japanese Laid-open Patent Publication No. 2005-6398.
However, to clamp the DC voltage that is input to the power circuit to the predetermined voltage, in the overvoltage protection circuit, a portion of micro energy generated by the AC power source is discarded to the ground etc. without being used. This deteriorates power generation efficiency. In particular, because the energy harvesting generates micro energy, it is undesirable to discard the generated energy without being used.